Elemental phosphorus can contain various heavy metals such as iron, arsenic, chromium, zinc and antimony at parts per million level. The amount of these contaminants present in the phosphorus is dependent upon the source of the phosphate ore from which the phosphorus is made and to some extent, the process used to refine the phosphorus, particularly the calcining of the phosphate ore prior to its entry into a furnace. For example, the levels of arsenic in white phosphorus can vary from 20 to about 220 parts per million (ppm) and antimony can range, for example, from 5 to about 70 ppm. The reduction of heavy metals in the phosphorus is desirable from an environmental viewpoint and it also can broaden the uses made of the phosphorus. For example, generally for food grade phosphoric acid both arsenic and antimony are preferably present at less than about 1 ppm. And for phosphoric acid used to make semiconductors or for other electronic uses, generally the arsenic is at less than about 50 parts per billion (ppb).
The literature provides many approaches to the removal of arsenic from elemental phosphorus. One approach is to wash elemental phosphorus with strong oxidants such as nitric or sulfuric acids or combinations of both, unexamined Japanese Patent application S52-160060(1979). This method is not desirable because of the large phosphorus loss, in the range of 20-50 percent due to oxidation by the acids, and large amounts of acid waste product which needs disposal. Distillation is another method of obtaining phosphorus with low arsenic levels, see for example, U.S. Pat. No. 4,483,746. Distillation is capital and labor intensive and also leaves a distillation residue product that is much higher in arsenic and other heavy metals due to the concentration caused by the distillation.
The disadvantages of washing elemental phosphorus with strong oxidants, or distillation or the formation of metal alloys is a premise for the addition of iodine or an iodine compound to phosphorus at 300.degree. C. or less to form arsenic iodide (AsI.sub.3) which has a much higher boiling point than phosphorus, as taught in unexamined Japanese Patent application HEI5(1993)43,210. This method enables the recovery of a larger portion of phosphorus due to the difference in boiling points and thus easier separation of phosphorus and the arsenic iodide. Although the distillation apparatus can be smaller and less expensive than that required to simply distill phosphorus, the process still has phosphorus losses in the distillation residue which also contains all of the removed arsenic and the iodine. Another disadvantage is the iodine must be added in great excess (about 10,000 times the stoichiometric levels).
Another approach to the avoidance of the various disadvantages of conventional processes for the removal of arsenic from phosphorus is the basis for unexamined Japanese Patent application H4(1994)-214796. The application discloses the use of iodine oxides, iodates, periodates and sodium hypoiodate as agents to treat phosphorus at 44-300.degree. C. to reduce arsenic levels. The arsenic is oxidized by the iodine compounds and becomes arsenous oxide making a distillation separation of the phosphorus and arsenic easier. The process still requires distillation, the use of 1,000 times the stoichiometric metric amount of iodine needed to react with the arsenic present and the yields varied from about 60-83 percent.
The iodate process of the present invention provides a cost effective means with a high yield (about 98 percent and better) of phosphorus to reduce the level of arsenic to less than about 5 ppm, and optionally the level of antimony (less than about 0.02 ppm) in elemental phosphorus. The arsenic and antimony are extracted and recovered in an aqueous phase. Any traces of iodine remaining with the phosphorus can be readily removed by washing the phosphorus with dilute hydrogen peroxide or other oxidant. The aqueous phase can be treated with known methods to remove the arsenic and antimony. Thus, the process of the present invention avoids a residue of arsenic (and antimony) in combination with phosphorus.